I made a replacement cowl from an original. I filled the original cowl with expanding foam to give it some rigidity. It was pretty beat up from use and that's why I needed a new one. After filling it with foam, I repaired all the outside surface with fiberglass to get a good plug. Sand and paint to get a smooth surface. Make a female mold from this plug. Layup your new cowl inside this mold.
I have documented a similar project (fiberglass canopy) on my web site:
http://webpages.charter.net/mcquinn/...s/projects.htm